Learning target: I can graph predator and prey records, calculate the cycle length and the lag between peaks, judge what the evidence can and cannot show about causes, and compare the pattern with real human population data.
Part A uses a teaching dataset, so the numbers are invented, but the pattern matches the real fur-trade records. Plot both animals on the same grid using two colors or two symbols, and connect each animal's points with a line. Then calculate. Parts B and C use real published results. Show every calculation and include units.
You will need: This worksheet, Pencil, Ruler, Two colored pencils, Calculator
For more than a century, trading posts across northern Canada recorded the snowshoe hare and Canada lynx pelts that trappers brought in. A pelt count tells you how many animals were trapped, which depends on how many animals there were but also on how many trappers were working and what furs were worth.
| Year | Hare pelts (thousands) | Lynx pelts (thousands) |
|---|---|---|
| 1 | 20 | 8 |
| 2 | 45 | 9 |
| 3 | 90 | 14 |
| 4 | 120 | 25 |
| 5 | 80 | 38 |
| 6 | 30 | 33 |
| 7 | 10 | 18 |
| 8 | 6 | 8 |
| 9 | 9 | 4 |
| 10 | 22 | 3 |
| 11 | 50 | 5 |
| 12 | 95 | 11 |
| 13 | 130 | 22 |
| 14 | 105 | 34 |
| 15 | 40 | 41 |
| 16 | 12 | 26 |
| 17 | 5 | 12 |
| 18 | 7 | 5 |
| 19 | 18 | 3 |
| 20 | 40 | 4 |
| 21 | 75 | 8 |
| 22 | 110 | 15 |
| 23 | 115 | 27 |
| 24 | 70 | 36 |
| 25 | 25 | 39 |
| 26 | 8 | 24 |
| 27 | 5 | 10 |
| 28 | 12 | 5 |
| 29 | 35 | 4 |
| 30 | 70 | 7 |
| 31 | 100 | 12 |
Pelt records show a pattern, but they cannot show what causes it. Ecologists have long debated whether hares crash mainly because predators catch up with them, because winter food (twigs and bark) runs out, or both. In the Yukon in the late 1980s and early 1990s, a team led by Charles Krebs changed conditions in separate square-kilometer blocks of forest: they fenced out mammal predators such as lynx and coyotes (owls and hawks could still fly in), added food, or did both, and compared each block with an unchanged block.
| Treatment | Hares compared with the unchanged block |
|---|---|
| Unchanged block (control) | 1 times |
| Mammal predators fenced out | about 2 times |
| Food added | about 3 times |
| Predators fenced out and food added | about 11 times |
| World population | Year |
|---|---|
| 1 billion | about 1804 |
| 2 billion | 1927 |
| 3 billion | 1960 |
| 4 billion | 1974 |
| 5 billion | 1987 |
| 6 billion | 1999 |
| 7 billion | 2011 |
| 8 billion | 2022 |
| about 8.2 billion | 2025 (estimate) |
| 9 billion (projected) | about 2037 |
| about 10.3 billion, the projected peak | mid-2080s |
| Years to add that billion |
|---|
| 1 → 2 billion |
| 2 → 3 billion |
| 3 → 4 billion |
| 4 → 5 billion |
| 5 → 6 billion |
| 6 → 7 billion |
| 7 → 8 billion |
| 8 → 9 billion (projected) |
Run your pencil down the hare column and circle each number that is bigger than both of its neighbors; those are the peaks. Do the same for lynx with a box. Then pair each hare circle with the next lynx box below it and subtract the years. For q8, subtract the earlier year from the later year in each pair of rows.
In 1973 an ecologist wrote a paper with that joking title, because in some stretches of the old fur records the hare peaks seem to come after the lynx peaks, which a simple predator-prey model cannot explain. One reason is that the famous hare series was stitched together from different regions and sources. Write a paragraph explaining how combining records from different places and different collection methods could create a misleading pattern, and propose a rule a scientist should follow before comparing two long-term data series.